The mechanism of heat transfer was analyzed by separating the heat transfer region in a rotary dryer into two parts : the period of material elevation on flights and the period of free fall from the flights. As a result, the volumetric heat transfer coefficient, U_a, for the rotary dryer was represented as a function of several dimensionless groups. The theoretical relationship thus obtained is discussed with respect to each parameter and compared with past experimental data. In particular, it is pointed out that there might be some questions in a conventional correlation method in which U_a is to be inversely proportional to the diameter of the rotary dryer.